Effects of intergrain sliding on crack growth in nanocrystalline materials

Theoretical models are suggested which describe the effects of intergrain sliding on crack growth in nanocrystalline metals and ceramics. Within the models, stress concentration near cracks initiates intergrain sliding which is non-accommodated at low temperatures and effectively accommodated at intermediate temperatures. The first model is focused on the non-accommodated intergrain sliding which leads to generation of dislocations at triple junctions of grain boundaries. These dislocations cause partial stress relaxation in the vicinities of crack tips and thereby hamper crack growth. It is shown that the non-accommodated intergrain sliding increases fracture toughness by 10–30% in nanocrystalline Al, Ni and 3C–SiC. The second model deals with the case of intermediate temperatures. Within this model, intergrain sliding is effectively accommodated by diffusion-controlled climb of grain boundary dislocations. The accommodated intergrain sliding in nanocrystalline materials results in crack blunting which, in its turn, leads to an increase (by a factor ranging from 1.1 to around 3, depending on temperature) of fracture toughness.